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JS Mfg.

Thought I'd put this up for those interested.
The only thing it lacks is a OBDI plug. Hope this helps anyone interested in making their own. Took me about an 1 1/2 hours after I had all the stuff. Linked right up with Datamaster (after I selected COM 1....DUH!)
I had to show the 7805 voltage regulator from the back in the schematic because I wanted to kinda squeeze everything together nicely without it getting confusing. Anytime you are following along a circuit and you see a "bump" as it crosses an intersecting line, that means there is no connection at that point.

Pin 13 and 14 on the MAX232 circuit have a little box next to them with numbers in them. Those numbers represent the pin that they need to run to on the 9 pin serial plug for your laptop.
The serial connector shows pin relation as if you just took the plug and just turned it up toward your face. Whenever you buy your 9 pin female serial plug, it will have the holes labeled on it, so no worries there.

I used 4 wire phone cord for the cables. I'd strip off the sheath and use the individual wires for jumpers on the board when I needed them. Many of the components had legs small enough, I could stick more than 1 leg in the same hole. REALLY cuts down on jumpers.

DO NOT reverse the polarity of your cigarette lighter plug. VERY important. If you are in doubt, plug the cord into your cigarette lighter and use a volt meter on the two bare ends. You'll have a black and a red probe. If you get a positive 12 volts on the meter, your red probe is on the positive lead. If you get a -12 volts on the meter, your black probe is on the positive lead. Make sure you label them so they don't get mixed up when you solder them to your board.
You don't have to use the two diodes but they helped eliminate the occasionally data errors I was getting.

After your build your interface, Datamaster actually has the ability to TEST your adaptor. Just go to the RECORD DATA screen and click on the "TEST COM" button. Also, if you forget to hook up your adaptor to your ALDL, it will tell you that the adaptor is working but you are getting no data. If you forget to plug in your cigarette lighter, it will tell you that no serial adaptor or ALDL signal present. Pretty cool.

There is a new MAX232 chip available that has no need for the 4 capacitors as they are internal. It is a MAX232A, I believe. Just do a Dogpile search and the specs will be easily found.

The two capacitors on the voltage regulator aren't absolutely necessary but eliminate "ripple". A big thanks to Barry Tisdale (the man's a GURU!!) for helping me out with the capacitor questions!!! The specs on the voltage regulators don't require anything near this high, but this is what I had. You can just use 1uF for all the capacitors in the circuit if that's all you have.

I'm gonna try to make some more. Kinda fun when you figure out what you are doing. My goal is to get it down to the size of the remote for the keyless entry.

This will probably be easier if I use a TO92 package instead of a TO-220 package for the voltage regulator and use a MAX232A chip.
Here's a link for the TO-92 packaged 5volt regulator if you are going for small size using the MAX232 setup:

There are NO connections on pins 7,8,9, or 10. Everything else must be hooked up as shown.

George Blake


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